1. Field of the Invention
This invention relates to a toroidal-type continuously variable transmission.
2. Prior Art
FIGS. 3 and 4 show one example of known toroidal-type continuously variable transmissions. An input disk 2 and an output disk 4 are rotatably supported on an input shaft 15 of a round tubular shape through respective needle roller bearings 16. A cam plate 10 is engaged with an outer peripheral surface of an end portion (left end portion in FIG. 3) of the input shaft 15 through splines, and is prevented by a flange portion 17 from moving away from the input disk 2. The cam plate 10 and rollers 12 jointly form a pressing device 9 of the loading cam-type which rotates the input disk 2 in accordance with the rotation of the input shaft 15, while pressing the input disk 2 toward the output disk 4. An output gear 18 is coupled to the output disk 4 by a key 19 so that the output disk 4 and the output gear 18 can synchronously rotate.
Power rollers 8 are held between the input disk 2 and the output disk 4, and the power rollers 8 are supported by a pair of trunnions 6 each swingable about pivot shafts 5 located in a twisted position relative to the input shaft 15. Opposite ends of each of the two trunnions 6 are supported respectively by a pair of support plates 20 in such a manner that the trunnion can be swung, and can be displaced in an axial direction (direction perpendicular to the sheet of FIG. 3; left-right direction in FIG. 4) A displacement-shaft 7 is supported in a support hole 23 of a circular shape formed in each trunnion 6. Each of the displacement shafts 7 has a support shaft portion 21 and a pivot shaft portion 22 which are parallel to each other, and are eccentric with respect to each other.
The support shaft portion 21 is rotatably supported in the support hole 23 through a radial needle roller bearing (first radial bearing) 24. The pivot shaft portion 22 projects from an inner surface 6a of the trunnion 6, and is inserted in an insertion hole 8b formed in the power roller 8. The power roller 8 is rotatably supported on the pivot shaft portion 22 through a radial needle roller bearing (second radial bearing) 25.
The pair of displacement shafts 7 are disposed respectively at diametrically-opposite (180 degrees spaced) positions with respect to the input shaft 15. The pivot shafts portions 22 of the displacement shafts 7 are eccentric with respect to their respective support shaft portions 21 in the same direction (opposite (right and left) directions in FIG. 4) with respect to the direction of rotation of the input and output disks 2 and 4. This direction of eccentricity is substantially perpendicular to the direction of extending of the input shaft 15. Therefore, each power roller 8 is supported in such a manner that it can be displaced slightly along the direction of extending of the input shaft 15. Therefore, even when each power roller 8 tends to be displaced in the axial direction of the input shaft 15 (that is, the left-right direction in FIG. 4; a direction perpendicular to the sheet of FIG. 5) due to variations in dimensional accuracy of the component parts, resilient deformation thereof and so on, this displacement can be absorbed without exerting an undue force on the component parts.
Provided between an outer surface of each power roller 8 and the inner surface 6a of the trunnion 6 are a thrust bearing 26 and a thrust needle roller bearing 27 (which supports a thrust load acting on an outer ring 30) which are arranged in this order from the outer surface of the power roller 8. The thrust bearing 26, while bearing a thrust load acting on the power roller 8, allows the rotation of the power roller 8.
The thrust bearing 26 comprises a plurality of balls (rolling elements) 29, and an annular retainer 28, holding the balls 29 in a manner to allow the rotation of these balls 29, and the annular outer ring 30. An inner ring raceway of the thrust bearing 26 is formed at the outer surface of the power roller 8, and an outer ring raceway thereof is formed at an inner surface of the outer ring 30.
Each thrust needle roller bearing 27 comprises a race 31, a retainer 32, and needle rollers 33. The race 31 and the retainer 32 are combined together in such a manner that they can be displaced slightly along the rotating direction. The thrust needle roller bearing 27 is held between the inner surface of the trunnion 6 and the outer surface of the outer ring 30, with the race 31 held in contact with the inner surface of the trunnion 6. The thrust needle roller bearing 27, while bearing a thrust load applied from the power roller 8 to the outer ring 30, allows the pivot shaft portion 22 and the outer ring 30 to swing about the support shaft portion 21.
A drive rod 36 is connected to one end portion (left end portion in FIG. 4) of each trunnion 6, and a drive piston 37 is fixedly mounted on an outer peripheral surface of this drive rod 36 intermediate opposite ends thereof. The drive piston 37 is fitted in a drive cylinder 38 in an oil-tight manner.
In the toroidal-type continuously variable transmission of the above construction, the rotation of the input shaft 15 is transmitted to the input disk 2 via the pressing device 9. Then, the rotation of this input disk 2 is transmitted to the output disk 4 via the pair of power rollers 8, and further the rotation of this output disk 4 is taken out from the output gear 18.
For changing the rotational speed ratio between the input shaft 15 and the output gear 18, the pair of drive pistons 37 are displaced in opposite directions, respectively. In accordance with the displacement of the pair of drive pistons 37, the pair of trunnions 6 are displaced in opposite directions, respectively, and for example the lower power roller 8 (FIG. 4) is displaced right (FIG. 4) while the upper power roller 8 is displaced left. As a result, the direction of a tangential force, acting on an area of contact between a traction surface 8a of each power roller 8 and an inner surface 2a of the input disk 2, as well as the direction of a tangential force acting on an area of contact between the traction surface 8a and an inner surface 4a of the output disk 4, is changed. As a result of this change of the direction of the force, the trunnions 6 are swung respectively in opposite directions about their pivot shafts 5 pivotally supported by the support plates 20. As a result, the position of contact between the traction surface 8a of each power roller 8 and the inner surface 2a (4a) is changed, so that the rotational speed ratio between the input shaft 15 and the output gear 18 is changed.
When the rotational force is thus transmitted from the input shaft 15 to the output gear 18, each power roller 18 is displaced in the axial direction of the input shaft 15 in accordance with the resilient deformation of the component parts, and each displacement shaft 7, pivotally supporting the power roller 8, is angularly moved slightly about the support shaft portion 21. As a result of this angular movement, the outer surface of the outer ring 30 of each thrust ball bearing 26 and the inner surface of the trunnion 6 are displaced relative to each other. Since the thrust needle roller bearing 27 is provided between this outer surface and this inner surface, a force, required for this relative displacement, is small. The force, required for changing the angle of inclination of each displacement shaft 7 as described above, is small.
In the toroidal-type continuously variable transmission, the transmission of the power between the input disk 2 (the output disk 4) and the power rollers 8 is thus effected by the traction drive. Therefore, it is necessary to apply a large pressing force to the point of contact (abutment) between the input disk 2 (the output disk 4) and each power roller 8. In order to produce this pressing force, there is, in many cases, used the above-mentioned pressing device 9 of the loading cam-type for producing the pressing force proportional to the input torque, a hydraulic pressing device (for producing an optimum pressing force) or the like.
When the input disk 2 is pressed toward the output disk 4 by the pressing device 9 upon driving of the toroidal-type continuously variable transmission, the pressing force and the traction force (tangential force) act on the point of contact of each power roller 8 with the input disk 2 and also on the point of contact of the power roller 8 with the output disk 4, so that a load acts on each power roller 8. Because of this load, the power roller 8 is deformed as indicated in broken lines in FIG. 5, so that the insertion hole 8b in the power roller 8 is also deformed.
Therefore, the inner peripheral surface of the insertion hole 8b in the power roller 8 is inclined with respect to the radial needle roller bearing 25, that is, the radial needle roller 25 is locally contacted with the inner peripheral surface of the insertion hole 8b. As a result, the resistance of the radial needle roller bearing 25 to the insertion hole 8b (that is, the rolling resistance of the radial needle roller bearing 25) increases, so that a power transmission loss at the radial needle roller bearing 25 increases.
And besides, as a result of deformation of the power roller 8, the inner ring raceway of the thrust bearing 26, formed at the outer peripheral portion of the power roller 8, is deformed. Therefore, a load, acting on the balls 29 of the thrust bearing 26, becomes uneven, so that the power transmission loss at the thrust bearing 26 increases.
In addition, when each displacement shaft 7 is inclined by the above traction force, a radial load acts on the balls 29 of the thrust bearing 26. Therefore, the smooth rotation of the balls 29 is prevented, so that the power transmission loss at the thrust bearing 26 increases.
Furthermore, a component of the above pressing force in the thrust direction acts on each trunnion 6, so that the trunnion 6 is resiliently deformed by this component. Therefore, the outer ring 30, supported on the trunnion 6 through the thrust needle roller bearing 27, is inclined, so that the load, acting on the balls 29 of the thrust bearing 26, becomes uneven, thus preventing the smooth rotation of the balls 29. And besides, the support hole 23 in the trunnion 6 is also deformed in accordance with the resilient deformation of the trunnion 6, and therefore the contact of the radial needle roller bearing 24 (provided in the support hole 23 in the trunnion 6) with the support shaft portion 21 of the displacement shaft 7 becomes uneven, so that the resistance of the radial needle roller bearing 24 to the support shaft portion 21 increases. Therefore, the power transmission loss at the radial needle roller bearing 24 provided in the support hole 23 in the trunnion 6, as well as the power transmission loss at the thrust bearing 26, increases.
Thus, in the conventional toroidal-type continuously variable transmission, the deformation of the power rollers 8, the inclination of the displacement shafts 7 and the deformation of the trunnions 6 occur at the time of driving this transmission, and therefore the smooth rotation of the bearings 24, 25 and 26 is prevented. As a result, the power transmission loss at each of the bearings 24, 25 and 26 increases, so that the power transmission efficiency is lowered.